Don Cothran M-Rod Power House Valve

[Motorhead]

Retractable bolt?

Is ... generally a design used in a Bolt Cocking PCP where the pellet or bullet when chambered is pushed in ahead of transfer port, then bolt and probe is drawn back to get behind transfer port.  In this way there is NO flow obstruction to the air delivered driving the pellet / bullet.

[Rattus58]

Retractable bolt?

Is ... generally a design used in a Bolt Cocking PCP where the pellet or bullet when chambered is pushed in ahead of transfer port, then bolt and probe is drawn back to get behind transfer port.  In this way there is NO flow obstruction to the air delivered driving the pellet / bullet.

Do you remember what they filed the bolt probe down to on the Marauder?

Much Aloha,

Tom

[Motorhead]

Retractable bolt?

Is ... generally a design used in a Bolt Cocking PCP where the pellet or bullet when chambered is pushed in ahead of transfer port, then bolt and probe is drawn back to get behind transfer port.  In this way there is NO flow obstruction to the air delivered driving the pellet / bullet.

Do you remember what they filed the bolt probe down to on the Marauder?

Much Aloha,

Tom

Filed ... LOL, there too hard to file !!
Grinder or belt sander you bet ...

[Rattus58]

Retractable bolt?

Is ... generally a design used in a Bolt Cocking PCP where the pellet or bullet when chambered is pushed in ahead of transfer port, then bolt and probe is drawn back to get behind transfer port.  In this way there is NO flow obstruction to the air delivered driving the pellet / bullet.

Do you remember what they filed the bolt probe down to on the Marauder?

Much Aloha,

Tom

Filed ... LOL, there too hard to file !!
Grinder or belt sander you bet ...

I was looking at a thread somewhere where the remedy was to put the bolt in a chuck and they used a file with a link to Amazon...  So how do you do this on a belt sander... chuck it in a drill and run it against a belt sander? and to what dimension in your opinion?

Much Aloha... Tom

[Hobbyman2007]

Rather than grinding down a bolt . I have and many others I'm sure, cut the probe off the end of the bolt and use a piece of drill bit (1/8") . The trick is getting the bolt probe length the same as before. I also use green locktite to secure the piece of drill bit in the hole.

[Motorhead]

Retractable bolt?

Is ... generally a design used in a Bolt Cocking PCP where the pellet or bullet when chambered is pushed in ahead of transfer port, then bolt and probe is drawn back to get behind transfer port.  In this way there is NO flow obstruction to the air delivered driving the pellet / bullet.

Do you remember what they filed the bolt probe down to on the Marauder?

Much Aloha,

Tom

Filed ... LOL, there too hard to file !!
Grinder or belt sander you bet ...

I was looking at a thread somewhere where the remedy was to put the bolt in a chuck and they used a file with a link to Amazon...  So how do you do this on a belt sander... chuck it in a drill and run it against a belt sander? and to what dimension in your opinion?

Much Aloha... Tom

Being the factory bolt / probe are hardened and therefor some what brittle, would not take it down to under 1/16" ( .063 )  The OEM .177 & .22 probes are already very close to this. ( Don't have one handy to measure )
Frankly taking / making the transfer port hole in barrel larger so long as under the 80% of bore diameter would gain area blocked by a larger probe stem with a smaller transfer hole.

[Rattus58]

So if I gained back some from the larger port diameter, I'd likely strike a medium gain, you're saying, with a more efficient valve. Are you also saying that I'd gain more efficiency beyond that if one used a smaller probe diameter and if so, any estimate as to what that gain might look like?

Thank you... Much Aloha,

Tom

[Motorhead]

So if I gained back some from the larger port diameter, I'd likely strike a medium gain, you're saying, with a more efficient valve. Are you also saying that I'd gain more efficiency beyond that if one used a smaller probe diameter and if so, any estimate as to what that gain might look like?

Thank you... Much Aloha,

Tom

No idea ... too many unknown variables.
Sadly PCP tuning is seldom so black & white in cause & effect.
Things that are known to be beneficial we do, end results vary much like a cooking recipe changing with slight variations of contents 

[Rattus58]

So if I gained back some from the larger port diameter, I'd likely strike a medium gain, you're saying, with a more efficient valve. Are you also saying that I'd gain more efficiency beyond that if one used a smaller probe diameter and if so, any estimate as to what that gain might look like?

Thank you... Much Aloha,

Tom

No idea ... too many unknown variables.
Sadly PCP tuning is seldom so black & white in cause & effect.
Things that are known to be beneficial we do, end results vary much like a cooking recipe changing with slight variations of contents

Well we'll take it a piece at a time and hope results are measurable enough to report...

[Sfttailrdr46]

So if I gained back some from the larger port diameter, I'd likely strike a medium gain, you're saying, with a more efficient valve. Are you also saying that I'd gain more efficiency beyond that if one used a smaller probe diameter and if so, any estimate as to what that gain might look like?

Thank you... Much Aloha,

Tom

No idea ... too many unknown variables.
Sadly PCP tuning is seldom so black & white in cause & effect.
Things that are known to be beneficial we do, end results vary much like a cooking recipe changing with slight variations of contents

Well we'll take it a piece at a time and hope results are measurable enough to report...

Report anyway that is how we all learn and it saves all us aspiring tuners a whole lot of missteps . That is why I just posted on the Hatsan Supercharger 25 that is on my work table in small pieces 

[Rattus58]

So if I gained back some from the larger port diameter, I'd likely strike a medium gain, you're saying, with a more efficient valve. Are you also saying that I'd gain more efficiency beyond that if one used a smaller probe diameter and if so, any estimate as to what that gain might look like?

Thank you... Much Aloha,

Tom

No idea ... too many unknown variables.
Sadly PCP tuning is seldom so black & white in cause & effect.
Things that are known to be beneficial we do, end results vary much like a cooking recipe changing with slight variations of contents

Well we'll take it a piece at a time and hope results are measurable enough to report...

Report anyway that is how we all learn and it saves all us aspiring tuners a whole lot of missteps . That is why I just posted on the Hatsan Supercharger 25 that is on my work table in small pieces 

Hehe... I'm predisposed that way it seems.... 

[int3man]

[/quote]Well we'll take it a piece at a time and hope results are measurable enough to report...
[/quote]

Hi Tom,
              humm Aloha!  I take a different approach.  I sleeve a .22 bolt.  I buy a replacement bolt from Crosman and install a sleeve to bring it up to .25 to seal the .25 Breech O-Ring.  Best of both works small probe Hardened.  If you want one PM me and we'll go from there.

Mahalo and Aloha!

[Rattus58]

Well we'll take it a piece at a time and hope results are measurable enough to report...
[/quote]

Hi Tom,
              humm Aloha!  I take a different approach.  I sleeve a .22 bolt.  I buy a replacement bolt from Crosman and install a sleeve to bring it up to .25 to seal the .25 Breech O-Ring.  Best of both works small probe Hardened.  If you want one PM me and we'll go from there.

Mahalo and Aloha!
[/quote]

Interesting... I'll PM you in the morning when I have my thoughts together... just got in (12:30AM) and just lookin over stuff having been away for a week.. thank you...

Much Aloha,

Tom

[DougGuy]

Getting ready to do some testing with the .308 barrel.
Here are a couple of side-by-side comparison pics of the .257 and the .308 barrels.  The .308 has a cross sectional area advantage .0745 sq in vs .0519 sqin, the .257 barrel is 30" long and the .308 is significantly shorter at 22.6" long.  The barrel ports on both barrels are almost identical, but the .308 is a little wider at .230 vs .185 for the .257.
The sleeve for the bolt is almost finished.
We'll see what happens.
Lloyd

In both photos, the .257 barrel is at the top of the picture.

Hi Lloyd,

I was skimming through this thread and thought of an idea that I would throw out there and see what you think of it.  I have the Cothran valve, I used a .375" brass port, .250" ID at the valve, going to a .187" x .309" slot in the barrel, and I used the brass so I could machine a smooth transition to the slot and not add extra plenum volume by using teflon tubing or PEEK.  Anyway, There is plenty enough wall thickess in the stock 25 caliber barrel to machine it deep enough to have room for a good seal and have barrel steel support all the way around instead of not breaking out on each side like the larger caliber barrels..  Mine is working good so far, and I'm good with sticking with a 25 caliber stock barrel, it is doing what I hoped it would do, Chrony results posted when I can get my fill tank valve repaired..

The idea I had, was to machine a T section out of carbon steel, that has the port made onto the bottom of it so it will reach down through the air tube and seal at the oring in the valve.  The top of the T could then be bored through, and the barrel threaded into one end of it, up to the port, and a custom sleeve threaded into the other end which would approximate the thickness of the barrel and allow the bolt probe to seal and also guide the pellet from the magazine to the breech.

This way, the seal from the valve to the bore is taken completely away from the top of the tport, and transferred to the sides of the barrel which would be much more effective, it would also open up the area where the port meets the barrel to having air enter the barrel 360 degrees, you would then overcome the reduction of plenum volume consumed by the bolt probe, and would use a standard unmodified bolt. and you machine out the "shroud" which would be the barrel extension between the port and the breech leaving only enough thin strips of metal to guide the pellet into the actual bore. 

If I can draw it up later I will make a sketch of it and edit this post and add it.

Ok please excuse this very crude sketch, but it gets the idea across, the idea I had was to use a T section to move the barrel seal to an area away from the top of the tport, and to open up the area above the tport so air could enter the barrel all around from a machined out pocket rather than just from the bottom side of the barrel..

The marauder breech block is 1" wide, and this could be made from a piece of 3/4" square stock, and the port could be machined from .375" steel rod, or 1/2" steel rod, then TiG welded into the bottom of the block.  That should leave enough meat in the aluminum breech block to machine out a 3/4" x 3/4? rectangular slot to insert the steel T block into.  There should be enough thickness to bore the steel block and thread it, and have a smaller smooth section between the threads and the port, for an o-ring placed on the end of the barrel to form a seal.

JUST an idea.....

[rsterne]

That is effectively a one-piece version of using a thimble that contains the barrel port....





The barrel slides into the front of the thimble, and seals against it with an O-ring.... The bolt slides into the back of the thimble, and seals on the internal O-ring.... On my thimble I use a Teflon transfer port in compression between the flat on the thimble and the flat on the top of the valve, and as long as you have about 0.010" of crush there are no leaks.... Obviously a one-piece, T-shaped thimble, would have one less point to leak.... It might be challenging to install in a receiver, unless it was one piece WITH the receiver though, no?.... I guess welding it in could work, but seems like a lot of work to eliminate one sealing point that isn't problematic anyway....

Bob

[DougGuy]

That is effectively a one-piece version of using a thimble that contains the barrel port....

The barrel slides into the front of the thimble, and seals against it with an O-ring.... The bolt slides into the back of the thimble, and seals on the internal O-ring.... On my thimble I use a Teflon transfer port in compression between the flat on the thimble and the flat on the top of the valve, and as long as you have about 0.010" of crush there are no leaks.... Obviously a one-piece, T-shaped thimble, would have one less point to leak.... It might be challenging to install in a receiver, unless it was one piece WITH the receiver though, no?.... I guess welding it in could work, but seems like a lot of work to eliminate one sealing point that isn't problematic anyway....

Bob

It would be the same principle as welding the tport to the spot face in your thimble, except the block would be inserted into the breech block from below instead of pushed into a bored hole.  The whole idea is to eliminate the seal point at the top of the tport, and to also allow air to enter the barrel from the end, instead of from a port in the bottom. 

I can see how your thimble effectively increases the diameter of the barrel, so to speak, so that a complete spot face is possible.  Clever way to overcome the radius of the barrel limiting the diameter of the seal..  Well done..

[rsterne]

The main purpose is to allow the barrel to be indexed.... Most barrels will shoot more accurately when the bend is towards the bottom (6 o-clock), some when it is towards the top (12 o'clock).... rarely in any other rotation.... In point of fact, my thimble OD and barrel OD were both 9/16".... although the receiver ended up drilled and tapped to 3/4"-16 for the barrel tensioning sleeve.... What you don't see in that photo is that the unthreaded portion is a collar threaded onto the barrel, and locked there with green Loctite, to take the end thrust of the sleeve....



The barrel can be rotated (indexed) into any position, and then locked there by tightening the large sleeve into the receiver....

Bob